In recent years, thermal transfer systems have been developed to obtain prints from pictures that have been generated electronically from a color camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to one of the cyan, magenta and yellow signals. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271, the disclosure of which is hereby incorporated by reference.
Thermal prints are susceptible to retransfer of dyes to adjacent surfaces and to discoloration by fingerprints. This is due to dye being at the surface of the dye-receiving layer of the print. These dyes can be driven further into the dye-receiving layer by thermally fusing the print with either hot rollers or a thermal head. This will help to reduce dye retransfer and fingerprint susceptibility, but does not eliminate problems. However, the application of a protection overcoat will practically eliminate these problems. This protection overcoat is applied to the receiver element by heating in a likewise manner after the dyes have been transferred. The protection overcoat will improve the stability of the image to light fade and oil from fingerprints.
Exposure of dyes to ultraviolet light (light with wavelengths less than 400 nm) usually results in degradation, or fading, of dyes with time. The degradation can be caused by photolysis, which is the direct absorption of the ultraviolet light. The dye can also degrade by either photooxidation or photoreduction depending on the chemical structure of the dye and of the natural or man-made polymer surrounding the dye. It is common to include a UV absorbing material in a heat-transferable protective overcoat layer for a dye-diffusion thermal transfer print as taught in U.S. Pat. No. 4,522,881 to reduce the rate of dye fading from ultraviolet light. Protection of the printed image from UV light should not degrade the stability of the image to daylight, which has a much lower intensity of UV light.
U.S. Pat. No. 6,184,375 suggests the use of triazine UV absorbers for a broad range of uses. It is suggested to employ such materials in various locations such as the receiver layer or in the overcoat as described in EP 505,734.
Copending commonly assigned U.S. Patent Application Publication US 2003/0176283A1, hereby incorporated by reference, discloses a heat transferred protective overcoat comprising a hydroxyphenyl triazine compound in a polymeric binder which overcoat has been placed over the image on a thermal dye sublimation receiving element and which overcoat provides UV protection without degrading daylight protection.
In a thermal dye transfer printing process, it is desirable for the finished prints to compare favorably with color photographic prints in terms of image quality. The look of the final print is very dependent on the surface texture and gloss. Typically, color photographic prints are available in surface finishes ranging from very smooth, high gloss to rough, low gloss matte.
The transferable protection layer of the dye donor that has a glossy finish is manufactured by a gravure coating process between the temperatures of 55° F. and 120° F., preferably between 65° F. and 100° F. A coating melt or solution is prepared from a solvent soluble polymer, a colloidal silica and organic particles and is transferred in the liquid state from the etching of the gravure cylinder to the dye donor support. The coated layer is dried by evaporating the solvent.
The transferable protection layer is usually one of at least two patches on the dye donor. It is transferred after printing an image from the dye donor to the surface of the dye-receiving layer of the receiver by heating the backside of the donor causing the transferable protection layer to adhere to the dye-receiving layer. The dye donor is peeled away from the receiver after cooling resulting in transfer of the protective layer. The surface of the transferred protective layer adhered to the dye-receiving layer has a measurable 60 degree gloss that is usually between 65 and 85 gloss units.
It has been found that the gloss on a laminated print decreases as the printing line time decreases, which is a problem as printing times become faster.